When an SEM emits an electron beam toward a surface, such incident beam produces backscattered electrons and secondary electrons. The number of backscattered and secondary electrons varies with the tilt angle, or slope, of the surface on which the beam is incident. It is expected that more secondary electrons will be emitted from a tilted surface than from a surface normal to the incident beam because with increased tilt a larger percentage of the beam interaction with the sample is close enough to the surface for electrons to be emitted.
In Chapter 4 of Scanning Electron Microscopy and X-Ray Microanalysis by J. Goldstein et al., Plenum Publishing 1992, the secondary electron emission is said to vary approximately as a secant function. If .theta. is the angle between the incident beam and a line normal to the sample surface, which makes .theta. the tilt angle, and because sec.theta.=1/cos.theta., according to this relationship the secondary electron emission varies as a reciprocal of cos.theta..
It would be valuable to use the SEM for determining the three-dimensional shape of a sample. In order to do so, however, it is necessary for the SEM to accurately measure the tilt angle of the sloped surfaces of the sample. If tilt were to be measured from the secondary electron emission determined in accordance with the relationship mentioned above, the result would be affected by the fact that it yields an emission of infinity at .theta.=90.degree.. If such a faulty relationship were to be relied upon to determine slope, incorrect tilt angles would be derived.